It is known in the art to join together two articles made of resins (and respectively opaque and transparent) by positioning them in contact with each other, transmitting a predetermined amount of laser beam focused on the junction of them and causing the junction portion to be melted and joined together (“laser welding”). Several advantages flow from laser welding versus conventional methods of joining plastic parts. For example, laser welding is widely known for its simple operation, labor savings, improvement of productivity, clear joints, and reduction of production cost. It is useful in various applications including the preparation of molded articles, including hollow shapes, in automobile industries and electric and electronic industries. Recently, work has intensified in the area of blends of thermoplastic resin and a colorant containing an organic dye or pigment. Better control of the conversion of laser energy to heat is achieved by the addition of such colorants to the resins. Laser beams penetrate through transparent articles positioned closer to the laser beam source, and are largely absorbed in the opaque article, which has a relatively higher absorption coefficient in comparison with the aforementioned transparent article. Careful attention to the amount of the colorants therein results in the junction portion being melted and the articles joined together.
See for example Japanese Published (Koukoku) Patent No. 62-49850 and Japanese Published (Koukoku) Patent No. 5 (93)-42336. Other resin compositions associated with the laser welding are described in U.S. Pat. No. 5,893,959 which discloses transparent and opaque workpiece parts welded together by a laser beam along a joining zone. Both parts contain black dye pigments such as carbon black to cause them to offer a substantially homogenous visual impression even after welding.
Other illustrations of the laser welding of compositions are found in U.S. Pat. No. 5,893,959. For example, the color of the thermoplastic components may be black (carbon black or nigrosine) which is commonly and widely used in the automobile industry among other applications. However, carbon black and nigrosine cannot transmit a laser beam with a main wavelength in the infra-red region (1200 nm to 800 nm), such as Nd:YAG laser and a diode laser, both of which are of wide use in industries.
Surprisingly, it has now been found that thermoplastic resin compositions both black in appearance can be used for laser-welded molded articles for both the transparent and opaque parts subjected to the laser beam. A significantly improved transmission to near-infrared light of the laser beam, with excellent and balanced heat-resistance and mechanical properties as required in automobile applications, is achieved by including a specific weight percentage of black dyes comprising amine salt of anthraquinone dyes.
Using these components, thermoplastic resin compositions can be utilized for laser welding and exhibiting improvements in moldability, solubility in the thermoplastic resin, bleeding- and blooming-resistance as well as transparency to the wavelength of a laser beam and resistance to chemicals.